Zeolite L catalyst for reforming

ABSTRACT

An improved reforming catalyst employing a zeolite L support is provided by soaking the zeolite L in an alkali solution having a pH of at least 11 for a time and at a temperature effective to increase the period of time over which the catalytic activity of the catalyst is maintained.

This is a division of application Ser. No. 426,211 filed Sept. 28, 1982now U.S. Pat. No. 4,448,891.

This invention relates to an improved zeolite L-based catalyst and theuse of this zeolite L-based catalyst in a reforming process, and moreparticularly to a zeolite L-based catalyst in which the catalyticactivity is maintained for an increased period of time. Further, thisinvention provides an improved reforming process in which the improvedzeolite L-based catalyst produces aromatic hydrocarbons at high yieldsand selectivity for an increased period of time.

The catalytic reforming reaction is conventionally used in the oilindustry for converting paraffins into aromatic hydrocarbons.Conventional methods of catalytic reforming are based on the use ofcatalysts comprising a noble metal on a carrier. Common catalysts ofthis kind are based on alumina carrying platinum and optionally a secondmetal such as rhenium. Use of carriers other than alumina, such as X andY zeolites have also been proposed provided the reactant and productsare sufficiently small to flow through the pores of the zeolites.

In conventional catalytic reforming processes hydrocarbons, preferablyC₆ to C₁₀ paraffins, are converted to aromatics by passing thehydrocarbons over the catalyst in the presence of hydrogen at operatingconditions generally within the temperature range of about 400° C. to565° C. and pressures varying from 200 kPa to 3.5 MPa at H₂ /feed moleratios of at least 3 and at LHSV of 0.2 to 5 W/W/HR. Part of thehydrocarbon feed is converted into aromatics by dehydrocyclization, butthe reaction is also accompanied by isomerization and hydrocrackingreactions. The latter converts some of the hydrocarbon feed, resultingin an undesirable loss of selectivity to C₄ -gaseous hydrocarbons. Thearomatic yield and selectivity of the reforming process varies with thereaction conditions and type of catalyst. High aromatic yield isdesired, but not at a disproportionate loss to C₄ -hydrocarbons, or at asignificant lessening of catalyst life.

One particularly suitable catalyst for reforming employs a type Lzeolite support which has shown to be more selective with regard to thedehydrocyclization reaction, thus providing a high yield of aromatichydrocarbons as disclosed in U.S. Pat. No. 4,104,320. However, oneproblem encountered in the use of a catalyst based on zeolite L is thatthe catalytic activity of the catalyst cannot be maintained for anextended period of time.

During the reforming process, the zeolite L-based catalyst isdeactivated fairly rapidly, for example, on the order of 50 to 150hours, generally by the agglomeration of the noble metal on the catalystand/or the formation of coke on the catalyst. While regeneration may becarried out, this is a time consuming and involved procedure whichrequires the reforming reactor to be shut down. Thus, economically acommercial reforming process typically requries the catalyst to beactive at high conversion rates for extended periods of time. For azeolite L-based catalyst it is desirable to maintain catalytic activityfor at least 250 hours.

Thus, a basic feature of this invention is to provide an improvedreforming catalyst employing a zeolite L support which maintains thecatalytic activity (high yield of aromatics) for an extended orincreased period of time.

SUMMARY OF THE INVENTION

Briefly, this invention provides an improved reforming catalystemploying a zeolite L support by soaking the zeolite L in an alkalisolution having a pH of at least 11 for a time and at a temperatureeffective to increase the period of time over which the catalyticactivity of the catalyst is maintained. The catalyst, based on zeolite Lwill have exchangeable cations of which at least 90% are alkali metalions selected from sodium, lithium, potassium, rubidium, barium, andcesium and containing at least one metal selected from Group VIII of theperiodic table of elements having a dehydrocyclizing effect. The alkalisoak is carried out prior to calcining the dehydrocyclizing metal loadedzeolite L.

BRIEF DESCRIPTION OF THE DRAWING

This invention will be better understood and its advantages will becomemore apparent from the following detailed description, especially whenread in light of the attached drawing wherein:

FIG. I is a chart of the results of Experiment I which compares underreforming conditions the benzene yield over time of an alkali soakedPt-K zeolite L catalyst versus an untreated Pt-K zeolite L.

DETAILED DESCRIPTION OF THE INVENTION

The improved catalyst and the resultant improved reforming process ofthis invention are achieved by soaking the zeolite L in an alkalisolution having a pH of at least 11 for a time and at a temperatureeffective to increase the period of time over which the catalyticactivity of the catalyst is maintained under reforming conditions. Thisalkali soak is carried out prior to calcining the dehydrocyclizing metalloaded zeolite L. The zeolite L support will be charged with one or moredehydrocyclizing metals to provide the reforming catalytic activity.

Type L zeolites are synthetic zeolites which crystallize in thehexagonal system with a characteristic X-ray diffraction spectrum. Atheoretical formula is M₉ /n[(AlO₂)₉ (SiO₂)₂₇ ]. The real formula mayvary by, for example, the ratio of silicon to aluminum varying from 2.5to 3.5. A general formula for zeolite L may be represented as follows:

    0.9-1.3M.sub.2/n O:Al.sub.2 O.sub.3 :5.0-7.0SiO.sub.2 :yH.sub.2 O

wherein "M" designates at least one exchangeable cation; "n" representsthe valence of "M"; and "y" may be any value from 0 to about 9. A morecomplete description of zeolite L is given in U.S. Pat. No. 3,216,789which more particularly gives a conventional description of zeolite Lwith respect to the X-ray diffraction spectrum. The zeolite L haschannel shaped pores and may occur in the form of cylindrical crystalsas well as other shapes. The crystals may be a thousand to fiftythousand angstroms in diameter. The hydrocarbon sorption pores arechannels parallel to the cylindrical axis and are between about 7 and 8angstroms in diameter. The K zeolite L as prepared is a basic zeolite,thus an alkali wash or ion exchange to reduce acidity is not required.

Type L zeolites are conventionally synthesized in a potassium form. Thecation is exchangeable so that, for example, a zeolite L in itspotassium form can be subjected to an ion exchange treatment in anaqueous solution of appropriate salts to provide a zeolite L containingother cations.

The reforming catalysts are based upon zeolite L in which at least 90%of the exchangeable cations are ions of at least one alkali metal chosenfrom potassium, barium, lithium, sodium, rubidium and cesium. In apreferred embodiment, the alkali metal cation is chosen from potassium,cesium and rubidium. Combinations of these ions may also be used. Forexample, a zeolite L in a potassium form can be ion exchanged bytreatment with an aqueous solution containing a rubidium and/or cesiumsalt, after which the zeolite is washed to eliminate excess ions. Thepercent of ions exchanged can be increased by repeating the ion exchangetreatment of the zeolite. However, since it is difficult to exchangemore than 80% of the original cation in the final product, the processyields zeolite L in which at least 90% of the exchangeable cations arepotassium ions and rubidium and/or cesium ions.

The zeolite L-based catalyst is charged with one or moredehydrocyclizing metals chosen from Group VIII of the periodic table ofelements, and optionally additionally containing rhenium, tin andgermanium. The preferred Group VIII metals are platinum, palladium andiridium, with the most preferred metal being platinum. In oneembodiment, platinum is combined with one of the metals of rhenium,iridium, tin or germanium to provide a desired reforming catalyst. Thelevel of the dehydrocyclizing metal will generally range from 0.1 to6.0% by weight of the catalyst. The level for platinum is 0.1 to 6.0%,preferably 0.2 to 5.0% by weight of the catalyst.

The dehydrocyclizing metals are introduced into the zeolite L support byimpregnation or ion exchange in an aqueous solution of the appropriatesalt. Platinum can be introduced on the support by the methods describedin the prior art, such as impregnation with an aqueous solution of asalt or of a platinum complex such as hexachloroplatinic acid,dinitrodiaminoplatinum or platinum tetramine dichloride or by depositthrough ion exchange with an aqueous solution of a platinum cationiccomplex like platinum tetramine dichloride. If more than one metal is tobe introduced in the catalyst, solutions containing salts of each metalmay be introduced simultaneously or sequentially. The zeolite L-basedcatalysts may further contain sulfur to increase selectivity bydecreasing C₄ -production in the subsequent reforming process.

The zeolite L-based catalyst as described above, especially thepreferred platinum-potassium zeolite L catalyst, exhibits very highyields of hydrocarbons to aromatics coupled with low rates of conversionof feed to C₄ -hydrocarbons. In a standardized test of a reformingprocess utilizing a 0.6 wt. % Pt-K zeolite L catalyst on a mixed C₆hydrocarbon feed, the yield of aromatics will initially be about 60weight % of the feed. However, the catalyst will deactivate fairlyrapidly, typically on the order of about 50 to 150 hours, depressing thearomatic (i.e. benzene) yield to below 40 weight % of the feed. With thealkali soak of this invention, while the initial catalytic activity andaromatic (benzene) yield may not be increased, the catalyst willmaintain its catalytic activity, i.e. yield to aromatics (benzene) of atleast 40 weight % of the feed for an increased period of time,preferably over 250 hours.

The zeolite L is soaked in an alkali solution having a pH of at least 11for a time and at a temperature effective to increase the period of timeover which the catalytic activity of the catalyst is maintained underreforming conditions. Generally, the alkali soak is effective toincrease the time by at least 50 hours over which the catalytic activity(aromatic yield) is maintained. Preferably, the alkali soak provides azeolite L-based reforming catalyst which under reforming conditions willconvert a mixed C₆ hydrocarbon feed to benzene at a yield of at least40% by weight of the feed for at least 250 hours, preferably at least300 hours. The above aromatic yields over time (catalyst life) are basedupon a standardized test utilizing a mixed C₆ hydrocarbon feedcontaining 60 weight % hexane, 30 weight % 3-methylpentane and 10%weight % methylcyclopentane and based on reforming conditions of about500° C., 690 KPa, LHSV of 2.5 W/W/HR and an H₂ /oil ratio of 6.

The degree of soaking of the zeolite L in an alkali solution to providean increase in catalyst life is dependent upon the variables of time,temperature and pH. With a lower pH, a longer soaking time and highertemperature will be required to provide the effective increase incatalyst life. The severity of the alkali soak is also dependent uponthe particular zeolite L, as different batches of zeolite L may vary inpurity (e.g. contamination with other structures) and particle size,thus requiring different degrees of alkali soak to effectively increasecatalyst life. To measure the effectiveness of the alkali soak on thezeolite L, the catalyt life (aromatic yield over time) of the resultingcatalyst is measured.

One effect of the alkali soak is that silica is preferentially removedfrom the type L zeolite. Thus, the alkali soak removes broadly from 0.05to 30% by weight of the silica and preferably removes from 0.1 to 15% byweight of the silica from the zeolite L, provided that substantial lossof crystallinity or destruction of crystal structure is avoided. Thealkali soak also decreases the SiO₂ /Al₂ O₃ ratio.

Another effect of the alkali soak is the NMR (nuclear magneticresonance) of the zeolite L is changed indicating a removal of silicafrom the crystal lattice without destruction of the crystal structure. Astill further effect is the X-ray diffraction shows the reduction ofimpurity zeolite W in the zeolite L material. Still further, the alkalisoak slightly changes the X-ray diffraction pattern of zeolite Lindicating an expansion by as much as 0.12 A° in the a_(o) latticedimension of 18.342 A°.

The alkali soak of this invention is substantially different from an ionexchange, a simple neutralization of residual acidity after calcining orreduction of acidity of a zeolite itself which is suggested by the art(e.g. see U.S. Pat. Nos. 3,216,789, 3,953,365, and 4,104,320). An ionexchange is generally carried out under mild conditions with minimaleffect on the support itself. The neutralization of residual acidityafter calcining is generally carried out under mild conditions simply toremove any residual acidity or hydrogen ions remaining after ionexchange and calcining with minimal effect on the support itself. Incomparison, the alkali soak of the instant invention is an extendedtreatment under severe conditions, which is critically carried out priorto calcining the dehydrocyclizing metal loaded zeolite L tofundamentally change the characteristics of the catalyst support itselfand provide a catalyst with increased life. The differences aredemonstrated in the preparation of zeolite L catalyst, in that whileneutralization of residual acidity after calcining is suggested (seeU.S. Pat. No. 4,104,320), the resultant zeolite L does not exhibitextended catalytic life. Further, the reduction of acidity of an acidiczeolite such as ZSM- 5 as suggested by European patent application No.80303065.9 would generally be inappropriate for zeolite L, as K zeoliteL as prepared is a basic zeolite.

The alkali soak is critically carried out at a pH of greater than 11,preferably at a pH of 13 to 16. Various alkalies that provide this pHmay be employed. Preferably the alkali has a cation chosen from sodium,potassium, rubidium, cesium, strontium and barium and has an anionchosen from hydroxide, carbonate, borate and phosphate, or combinationsthereof. The preferred alkalies are potassium hydroxide, rubidiumhydroxide, cesium hydroxide or combinations thereof. While preferablythe alkali solution is an aqeuous solution, other solvents such asalcohol, alcohol and water etc. may be employed. The interdependentvariables of pH, temperature and time of soaking must be controlled toprovide the desired effect on the catalyst. Generally, the soaking iscarried out under continuous agitation and the temperature of thesoaking will range between -20° to 200° C., preferably 25° to 120° C.,and the time will vary between 15 minutes to three weeks, with thepreferred time period being five hours to eighty hours. The soakingitself can be carried out as one continuous extended soak or as a seriesof soaks, followed by washing. Once the alkali soak of the desireddegree is achieved, then the zeolite L is washed, preferably in thesolvent to remove excess ions, followed by drying the zeolite L.

The alkali soak can be carried out on the zeolite L before or after itis charged with the dehydrocyclizing metal. However, in order to avoidloss of this dehydrocyclizing metal which can be expensive in the caseof platinum, the alkali soak is preferably carried out prior to thecharging of the dehydrocyclizing metal on the zeolite L support.Critically, the alkali soak is carried out on the zeolite L before thedehydrocyclizing metal loaded zeolite L is calcined to activate thecatalyst. After calcining, the alkali soak does not effectively extendthe catalyst life. The zeolite L is preferably shaped into anappropriate size and shape for subsequent use as a reforming catalyst.The shaping can be carried out with or without a binder, with care beingtaken in that the binder chosen should not detrimentally affect thereforming process by substantially reducing the catalytic activity. Thealkali soak is preferably carried out without a binder present to insurethe desired effect on the zeolite L itself, versus simply neutralizingthe acidity of the binder.

The zeolite L based catalyst as improved by the alkali soak is thensuitable for use for an extended period of time in a reforming process.In general, the reforming process is carried out at: temperatures of400° C. to 550° C., preferably 450° C. to 520° C.; pressures of 200K Pato 2M Pa, preferably 517K Pa to 1.2M Pa; a liquid hourly space velocity(LHSV) of 0.5 to 20 W/W/HR, preferably 1 to 10 W/W/HR; and at H₂ /oilmole ratios of 2 to 20, preferably 4 to 10. While various hydrocarbonfeeds may be used, the preferred feed contains paraffins having 6 to 10carbon atoms.

A number of catalyst were prepared and tested under reforming conditionsin the following examples. The standard reforming conditions employed inthe examples involved utilizing a mixed C₆ hydrocarbon feed containing60 weight % normal hexane, 30 weight % 3-methylpentane and 10 weight %methylcyclopentane, and reforming conditions comprising a temperature ofabout 500° C., pressures of 690 kPa (100 psig), LHSV of 2.5 grams feedper hour per gram of catalyst and at an H₂ /oil mole ratio of 6. Usefulcatalyst life for these experiments was defined as the time periodrequired to maintain a benzene yield of at least 40% by weight of thefeed. The catalysts tested were platinum-potassium zeolite L with thelevel of platinum indicated (% by weight of catalyst) being loaded onthe zeolite L support, after any alkali soak, by ion exchange withPt(NH₃)₄ Cl₂ dissolved in water, followed by drying and then calciningat 480° C. for three hours.

EXAMPLE I

Prior to loading the platinum on zeolite L, 30 grams of K zeolite L wassoaked in 200 cc of an aqueous solution containing 20 weight % of KOH ata pH of about 14.5. The slurry was continually stirred while thetemperature of the solution was maintained at 55° C. for 18 hours. Thealkali soaked zeolite was then washed with water followed by repeatingthe soaking in the KOH solution for two additional 18 hour periods, fora total alkali soaking time of 54 hours. Washing was then repeated untilthe pH of the zeolite wash water was at or below 10.5, followed bydrying at 110° C. The alkali soaked K zeolite L was then loaded with 0.6wt. % platinum and compared under standard reforming conditions with amixed C₆ hydrocarbon feed against 0.6 wt. % Pt-K zeolite L which has notbeen alkali soaked. Table I lists the benzene yield in weight % of feedover time for the alkali soaked Pt-K zeolite L and the untreated Pt-Kzeolite L, the results of which are plotted in FIG. 1.

The 0.6 wt. % Pt-K zeolite L which was alkali soaked demonstrated auseful catalyst life (i.e., a benzene yield of at least 40 weight % ofthe feed) for about 265 hours as compared to about 88 hours for theuntreated 0.6 wt. % Pt-K zeolite L.

                  TABLE I                                                         ______________________________________                                        Untreated          Alkali Soaked                                              0.6 wt. % Pt-K Zeolite L                                                                         0.6 wt. % Pt-K Zeolite L                                   Time     Benzene       Time    Benzene                                        (hrs)    Yield (wt. %) (hrs)   Yield (wt. %)                                  ______________________________________                                        0.25     51.7          .25     46.6                                           1        60.3          1       61                                             18       57.8          19      61.6                                           25       56.8          26      61.8                                           42       53.5          43      60.1                                           49       53            49      60.8                                           66       47.9          67      59.4                                           73       47.3          73      58.7                                           118      28.2          119     53.9                                           162      18.7          163     51.6                                           186      14.4          187     49.4                                           193      13.1          193     49                                             217      11.5          217     47.6                                           235      11.0          236     43.9                                           242      3.8           242     44.9                                           263      6.4           263     40.7                                                                  265     40.5                                                                  332     32.4                                           ______________________________________                                    

EXAMPLE II

A zeolite L base was alkali soaked as specified in Example I, thenloaded with 1.0 wt. % platinum and compared under standard reformingconditions with a mixed C₆ hydrocarbon feed against an untreated 1.0 wt.% Pt-K zeolite L. Table II lists the benzene yield in weight % of feedover time for the alkali soaked Pt-K zeolite L and the untreated Pt-Kzeolite L.

The 1.0 wt. % Pt-K zeolite L which was alkali soaked demonstrated auseful catalyst life of about 285 hours as compared to about 142 hoursfor the untreated 1.0 wt. % Pt-K zeolite L.

                  TABLE II                                                        ______________________________________                                        Untreated          Alkali Soaked                                              1.0 wt. % Pt-K Zeolite L                                                                         1.0 wt. % Pt-K Zeolite L                                   Time     Benzene       Time    Benzene                                        (hrs)    Yield (wt. %) (hrs)   Yield (wt. %)                                  ______________________________________                                        .25      55.4          0.25    10.7                                           1        63.8          1       45.6                                           19       62.5          2       48.4                                           26       59.7          19      55.5                                           43       57.3          25      55                                             50       55.5          43      54.4                                           115      44.1          95      52                                             121      42.7          139     49.7                                           139      40.4          145     49.2                                           146      39.7          164     48.6                                           165      36.4          187     46.6                                           166      37            214     44.9                                                                  235     43.4                                                                  237     42.8                                                                  307     38.2                                                                  308     38.8                                           ______________________________________                                    

EXAMPLE III

Prior to loading the platinum on zeolite L, 30 grams of K-zeolite L(same K-zeolite L as used in Example I) was soaked in 200 cc of anaqueous solution containing 10 weight % of KOH at a pH of about 14.2.The slurry at room temperature (25° C.) was continuously stirred for 3days. The soaked zeolite was repeatedly washed in water to remove excessKOH until the pH of the wash water was at 10.5 or below. The soakedzeolite was then loaded with 0.6 wt. % platinum and tested understandard reforming conditions with a mixed C₆ hydrocarbon feed. TableIII lists the benzene yield in weight % of feed over time for the alkalisoaked zeolite.

The alkali soaked 0.6 wt. % Pt K zeolite L demonstrated a usefulcatalyst life of about 350 hours as compared to about 88 hours for theuntreated 0.6% Pt K zeolite L (see Example I).

                  TABLE III                                                       ______________________________________                                        Alkali Soaked                                                                 0.6 wt. % Pt Zeolite L                                                               Time  Benzene                                                                 (hrs) Yield (wt. %)                                                    ______________________________________                                               .3    50.7                                                                    1     55.2                                                                    19    59.8                                                                    25    58.2                                                                    44    58.1                                                                    67    57.7                                                                    142   54.7                                                                    164   53.3                                                                    187   54.1                                                                    194   53                                                                      211   52.1                                                                    235   51.8                                                                    260   48.4                                                                    331   43.1                                                                    338   42.1                                                                    348   40.6                                                                    353   39.4                                                                    371   37.5                                                                    373   37.8                                                             ______________________________________                                    

EXAMPLE IV

Prior to loading the platinum on zeolite L, 50 grams of K zeolite L wassoaked in 200 cc of an aqueous solution containing 50 weight % of KOH ata pH of about 15. The slurry was heated to 110° C. and stirredcontinuously for 18 hours. The soaked zeolite was repeatedly washed inwater until the pH of the wash water was at 10.5 or below. The alkalisoaked zeolite was then loaded with 1.0 wt. % platinum and tested understandard reforming conditions with a mixed C₆ hydrocarbon feed. Table IVlists the benzene yield in weight % of feed over time for the untreatedand the alkali soaked zeolite L.

The alkali soaked 1.0 wt. % Pt K zeolite L demonstrated a usefulcatalyst life of about 190 hours as compared to 0 hours for an untreated1.0 wt. % Pt K zeolite L.

                  TABLE IV                                                        ______________________________________                                        Untreated          Alkali Soaked                                              1.0 wt. % Pt K-Zeolite L                                                                         1.0 wt. % Pt K-Zeolite L                                   Time     Benzene       Time    Benzene                                        (hrs)    Yield (wt. %) (hrs)   Yield (wt. %)                                  ______________________________________                                        2.5      32%           .25     50.1                                           19       28%           1       53.8                                           20       28%           18      52.9                                                                  24      53                                                                    42      51                                                                    48      51.4                                                                  138     42                                                                    145     42.9                                                                  162     41.7                                                                  168     42.4                                                                  190     39.8                                                                  192     39.1                                           ______________________________________                                    

EXAMPLE V

A zeolite-L support (same K-zeolite L as used in Example IV) was alkalisoaked as specified in Example I, then loaded with 1.0 wt. % platinumand compared under standard reforming conditions with a mixed C₆hydrocarbon feed against an untreated 1.0 wt. % Pt-K zeolite L. Table Vlists the benzene yield in weight % of feed over time for the alkalisoaked catalyst.

The 1.0 wt. % Pt-K zeolite L which was alkali soaked demonstrated auseful catalyst life of about 535 hours as compared to 0 hours for theuntreated 1.0 wt. % Pt-K zeolite L (see Example IV).

                  TABLE V                                                         ______________________________________                                        Alkali Soaked                                                                 1 wt. % Pt-K Zeolite L                                                               Time Benzene                                                                  (hrs)                                                                              Yield (wt. %)                                                     ______________________________________                                               .25  16.7                                                                      1   57.5                                                                      19  60.5                                                                      23  60.6                                                                      44  60.8                                                                      68  59.4                                                                      91  58.9                                                                      96  59.4                                                                     164  58                                                                       188  57.6                                                                     212  57.5                                                                     236  55.5                                                                     259  54.7                                                                     331  52.6                                                                     355  52.6                                                                     379  50.4                                                                     403  49.7                                                                     499  43.7                                                                     523  42.3                                                                     528  42.7                                                                     548  38.9                                                                     553  39.2                                                                     572  36.9                                                              ______________________________________                                    

EXAMPLE VI

After loading 0.6 wt. % of platinum on zeolite L (same K-zeolite L asused in Example I), the zeolite L was dried and then calcined at 480° C.for three hours. Following calcining, 10 grams of the 0.6 wt. % Pt-Kzeolite L was alkali soaked in 70 cc of an aqueous solution containing10 wt. % KOH at a pH of about 14.2. The slurry was continually stirredwhile the temperature was maintained at 25° C. for 3 days. The alkalisoaked zeolite was then washed until the pH of the wash water was at10.5 or below, followed by drying. Table VI lists the benzene yield inweight % of feed over time for the zeolite L which was alkali soakedafter calcining the platinum loaded zeolite L.

The 0.6 wt. % Pt-K zeolite L which was alkali soaked after calcining thedehydrocyclizing metal (platinum) loaded zeolite L demonstrated a usefulcatalyst life of about 3 hours as compared to about 88 hours for theuntreated 0.6 wt. % Pt-K zeolite L (see Example I) and as compared to analkali soak prior to calcining the 0.6 wt. % Pt-K zeolite L whichdemonstrated a useful catalyst life of about 350 hours (see ExampleIII). Thus, no improvement in useful catalyst life is shown for alkalisoaking after calcining the dehydrocyclizing metal loaded zeolite L.

                  TABLE VI                                                        ______________________________________                                        Alkali Soaked After Calcining                                                 0.6 wt. % Pt-K Zeolite L                                                             Time  Benzene                                                                 (hrs) Yield (wt. %)                                                    ______________________________________                                               .25   55%                                                                     1.25  53%                                                                     2     47%                                                                     4     34%                                                                     5     39%                                                              ______________________________________                                    

What is claimed is:
 1. A reforming process for producing aromatichydrocarbons from a hydrocarbon feedstock containing paraffins of atleast C₆ comprising: contacting the hydrocarbon feedstock plus addedhydrogen at elevated temperatures with a catalyst which comprises azeolite having exchangeable cations of which at least 90% are metal ionsselected from the group consisting of sodium, lithium, potassium,rubidium, barium and cesium and containing at least one metal selectedfrom the group consisting of Group VIII of the periodic table ofelements and having a dehydrocyclizing effect, said zeolite produced bysoaking a type L zeolite prior to calcining the dehydrocyclizing metalloaded zeolite in an alkali solution having a pH of at least 11 for atime and at a temperature effective to increase the period of time overwhich the catalytic activity of the catalyst is maintained underreforming conditions.
 2. Process of claim 1 wherein the alkali soak iscarried out to preferentially remove silica from the type L zeolitewithout substantial loss of crystallinity.
 3. Process of claim 2 wherein0.1 to 15 weight % of the silica is removed from said zeolite. 4.Process of claim 1 wherein the alkali soak is effective to increase byat least 50 hours the time period over which catalytic activity ismaintained, based on a standardized test utilizing a hydrocarbonfeedstock containing 60 weight % hexane, 30 weight % 3-methylpentane,and 10 weight % methylcyclopentane and on reforming conditions of about500° C., 690 KPa, space velocity of 2.5 W/W/HR and an H/₂ oil mole ratioof
 6. 5. Process of claim 1 wherein the alkali of the alkali solutionhas a cation chosen from the group consisting of sodium, potassium,rubidium, cesium, strontium, barium and mixtures thereof and has ananion chosen from the group consisting of hydroxide, carbonate,phosphate, borate and mixtures thereof.
 6. Process of claim 5 whereinthe alkali solution is an aqueous solution.
 7. Process of claim 6wherein the soaking is carried out under agitation, the pH of the alkalisolution is 13 to 16, the temperature of soaking is between 25° and 120°C. and the soaking time is between 2 and 80 hours.
 8. Process of claim 7wherein the alkali is chosen from the group consisting of potassiumhydroxide, cesium hydroxide and rubidium hydroxide.
 9. Process of claim8 further comprising washing the soaked zeolite to remove excess alkali,followed by drying the zeolite.
 10. Process of claim 1 wherein thezeolite catalyst contains at least one alkali metal selected from thegroup consisting of potassium, rubidium and cesium and contains from 0.1to 6.0% by weight of at least one dehydrocyclizing metal selected fromthe group consisting of platinum, palladium and iridium.
 11. Process ofclaim 10 wherein the zeolite additionally contains at least one metalchosen from the group consisting of rhenium, tin and germanium. 12.Process of claim 10 wherein the zeolite contains potassium and 0.2 to 5%by weight of platinum.
 13. Process of claim 10 wherein the zeolitefurther contains sulfur.
 14. Process of claim 1 wherein the zeolite ischarged with the dehydrocyclizing metal after the alkali soak. 15.Process of claims 1, 4, 8, 10 or 14 further comprising shaping thesoaked zeolite.
 16. Process of claim 1 wherein the reforming conditionscomprise temperatures of 450° to 520° C.; pressure of 517 KPa to 1.2MPa; hourly liquid space velocity of 1 to 10 W/W/HR; and H₂ /oil moleratio of 4 to
 10. 17. Process of claims 1, 3, 7, 8 or 12 wherein thealkali soak is effective to increase the time to at least 250 hours overwhich the catalytic activity of the catalyst is maintained at a level ofat least a 40 weight % conversion to aromatics, based on a standardizedtest utilizing a hydrocarbon feedstock containing 60 weight % hexane, 30weight % 3-methylpentane and 10 weight % methylcyclopentane and onreforming conditions of about 500° C., 690 KPa, space velocity of 2.5W/W/HR and an H₂ /oil mole ratio of
 6. 18. Process of claim 17 whereinthe alkali soak is effective to maintain catalytic activity for at least300 hours.